1. Field of the Invention
The present invention relates to communications systems having mobile subscribers. More particularly, the present invention relates to a method of routing calls to such mobile subscribers under certain circumstances.
2. Description of Related Art
In certain communication systems, such as in some radiotelephone systems, the whole service area of the systems are divided into a plurality of local service areas. In each local area, radio access units are located at one or more key points to establish radio links between mobile subscribers and mobile switching centers. Mobile subscribers are permanently assigned unique telephone numbers recognized by the public switched network and they are also temporarily assigned a different number whenever they roam into another local area. Assignment of such a temporary subscriber number requires that the system be provided with information on the current location of each mobile subscriber. Each of the mobile subscribers of the system must therefore be registered exclusively in one of the local areas. To maintain this exclusivity, whenever a mobile subscriber moves to another local area, the old registration must be cancelled in the area departed and a new registration made in the area in which the mobile subscriber just arrived.
Heretofore, whenever a subscriber with automatic roaming capabilities was first detected in a cooperating exchange, the subscriber was immediately registered as an automatic visitor within the exchange. Upon registration, the subscriber was allocated a subscriber number. This number was employed to assist an interrogation exchange or the visitor's home exchange in routing a call to the subscriber.
If all calls were only routed within a cooperating exchange network, the numbers could be assigned by the exchange itself. However, these numbers would not be recognized by other systems (i.e., the Public Switched Telephone Network). To accommodate calls that require routing outside the cooperating network, subscriber numbers often have to be leased from an outside source.
It should also be recognized that heretofore, the subscriber numbers allocated at connection of the automatic visitor have not been released until the visitor has disconnected. As a result, it has been necessary to have as many subscriber numbers as there are registered automatic visitors. This has been to ensure that each registered automatic visitor has the capability of receiving a call. Needless to say, however, leasing subscriber numbers from an outside source can be expensive. As prior art systems have uniformly required at least one leased number per registered automatic visitor, these prior art systems have all been implemented at relatively high cost. Based upon these facts, it is clear that lacking a method of reducing the number of leased subscriber numbers in prior art communication systems is a deficiency and shortcoming of the prior art.
Although there are no known prior art teachings of a solution to the aforementioned deficiency and shortcoming such as that disclosed and claimed herein, a number of prior art references exist that discuss subject matter that bears some relation to matters discussed herein. Such prior art references are U.S. Pat. Nos. 4,228,319, 4,612,415, 4,748,655, 4,794,635, 4,833,701 and 4,833,702. Each of these references is discussed briefly below.
U.S. Pat. No. 4,228,319 to De Jager et al. relates to an automatic mobile subscriber system. De Jager et al. teach stationary stations which repeatedly transmit through their cell channel a channel number of a free duplex speech channel which is stored in a storage device in mobile stations receiving the call channel. De Jager et al. also teach use of the stored channel number for automatically selecting the free duplex speech channel in response to a connection set-up command.
U.S. Pat. No. 4,612,415 to Zdunek et al. relates to a method and means for controlling telephone interconnect traffic in a trunked radio system. Zdunek et al. teach continuously monitoring all types of communication traffic on the system and, in response to an increasing dispatch access delay, reserving certain repeaters for dispatch use only during a predetermined period. Zdunek et al. also teach a system in which the number of simultaneous telephone interconnect calls permitted on the system during the predetermined period is dynamically altered in response to system loading. Still further, Zdunek et al. teach establishment of a variable, periodically updated, maximum interconnect call length based on the current system dispatch access delay.
U.S. Pat. No. 4,748,665 to Thrower et al. relates to portable telephones. According to the teachings of Thrower et al., a personal portable telephone allows a user access to a cellular radio network via a number of different gateways including a communal unit, user's mobile telephone set, home telephone or office PABX. The telephone can also be used as a paging unit for use within the cellular radio network. The communal radio unit is a multi-channel arrangement capable of servicing a large number of personal telephones within its service area which is small compared to that of a normal cellular radio cell. The communal units may be transportable and located in public areas such as railway stations, airports and on trains or coaches. The telephone automatically transmits its identification number to a gateway when it enters its service area enabling the subscriber to be accessed by callers without their knowing his whereabouts.
U.S. Pat. No. 4,794,635 to Hess teaches a two-way radio communication system having a limited number of channels which assigns calls in a max-minimum method. Hess's method also ascertains the type of call, handoff or first assignment, and assigns a server with a given central controller to a channel frequency within the requestor sector that minimizes interference to present users. In a second embodiment of his method, Hess teaches requesting assignment to a limited number of channels to reduce adjacent channel interference to and from the requestor with respect to present users operating on a system by utilizing one or more thresholds to arrange channel assignments based upon progressively larger or smaller values of received signal strength.
U.S. Pat. No. 4,833,701 to Comroe et al. relates to a trunked communications system with nationwide roaming capability. According to the teachings of Comroe et al., selected regional trunking systems are equipped with telephone interchange capability and provided with local computers, which communicate with a national hub computer. At each selected trunking system, several ID's are reserved as "roaming ID's" to be temporarily assigned to roaming subscribers. When a subscriber determines that he has roamed into a new trunk system, he requests a roaming ID. A roaming ID is assigned and transmitted to the subscriber, which thereafter operates within the new trunked system using its roaming ID. The roaming assignment is also transmitted to the national hub computer so that interconnect calls may be properly forwarded. This differs from the present invention, wherein a roaming number assignment is requested at each call attempt rather than transmitted to the national hub computer. Referring again to Comroe et al.'s teachings, the roaming subscriber continues to operate under the assigned roaming ID until it roams out of range of the current system and into yet another trunked system. In this way, the subscriber may roam from system to system.
U.S. Pat. No. 4,833,702 to Shitara et al. relates to telephone registration and cancellation control in a wide area cordless telephone system. Specifically, Shitara et al. teach a method for making a registration of a roaming cordless telephone in one of a plurality of local service areas and cancelling the registration.
Review of each of the foregoing patents reveals no disclosure or suggestion of a system or method such as that described and claimed herein.